In an internal combustion engine a piston moves between two positions, an upper and a lower turning point. An inlet stroke is followed by a compression stroke that is followed by a work stroke which in turn is followed by an exhaust stroke ending the procedure, so to say a standard 4-stroke procedure. In an ordinary diesel engine the inlet and outlet valves open with the aid of a camshaft. Further the ratio between the cylinder volume and the combustion chamber volume, the compression ratio, is constant e.g. 16:1 or 17:1 or otherwise. The high compression ratio causes a high efficiency.
A problem to be solved in the current diesel engines is to reduce NOx which is the result of a mixture of oxygen and nitrogen in air and is increasing rapidly with raising combustion temperatures. Combustion at high temperatures reduces both the HC and CO as well as the creation of particles and contributes to a reduction in fuel consumption. On the other hand, there is increase in NOx which is unwanted because of the health risks associated with inhaling, particularly in city traffic.
It is known that the NOx is produced at high temperatures and during lean conditions, i.e. little fuel and much air, so-called air surplus.
Diesel engines use the so-called qualitative combustion which means that in principle the same volume of air is compressed cycle after cycle and the varying needs of power is provided by injecting varying amounts of fuel. As a result of this the often more or less lean conditions occur with more or less air excess, which leads to the formation of more or less NOx. Qualitative combustion thus forms the different amounts of NOx in the combustion gases depending on the current engine load and this is precisely what makes it difficult to find a method to continuously reduce NOx. The problem can be summarized to that qualitative combustion leads to varying air excess and the accompanying formation of NOx which today often tries to be reduced by the so-called EGR (EXHAUST GAS RECIRCULATION) recirculation of the cooled exhaust gases to the inlet.